Heat exchangers and cores and extended surface elements therefor



June 30, 1959 s, HOLM 2,892,618

HEAT EXCHANGERS AND coREs AND EXTENDED SURFACE ELEMENTS THEREFOR FiledApril 12, 1957 2 Sheets-Sheet l INVENTOR. JVE/V //04M June 30; 1959 s.HOLM 2,392,618

HEAT EXCHANGERS AND CORES AND EXTENDED SURFACE ELEMENTS THEREFOR FiledApril 12, 1957 2 Sheets-Sheet 2 United States Patent HEAT EXCHANGERS ANDCORES AND EX- TENDED SURFACE ELEMENTS THEREFOR Sven Holm, Hudson, Ohio,assignor to Ferrotherm Company, Cleveland, Ohio, a corporation of OhioApplication April 12, 1957, Serial No. 652,527

6 Claims. (Cl. 257-245) This invention relates to heat exchangers, andcores and extended surface elements therefor, and is an improvement onthe heat exchangers, cores, and pin fin extended surface elementsdisclosed in US. Patent No. 2,678,808, issued May 18, 1954, to J. R.Gier, In, and entitled Sinuous Wire Structural and Heat ExchangeElements and Assemblies.

As more fully described in the above identified patent, relativelyefiicient heat exchangers can be made by incorporating between suitablemetal plates a myriad of metal pins of small diameter arranged inparallel rows and spaced apart from each other a short distance in therows which, in turn, are spaced apart from each other transversely ofthe rows about the same distance as a pair of adjacent pins of the samerow. The pins of each row are formed of a single length of wire which atportions spaced along its length, is flattened to form ligaments whichare very thin and are readily bendable at their junctures with theintermediate portions of the wire, or legs, therebetween, so that theformed wire can be bent into sinusoidal or corrugated form in which theintermediate portions, or legs, are parallel to, and in radially spacedrelation from, each other.

The pins may have the normal cross-section of the length of wire fromwhich they are formed or a crosssection modified so that each pin isstreamlined in the direction of flow, whether it be along the row ofpins or transversely of the rows.

The present heat exchange elements are elements which can be formed of asingle length of wire in the manner described in the above identifiedpatent and in the United States application Serial No. 342,823, filedMarch 17, 1953, now Patent No. 2,778,385 granted January 22, 1957, by J.R. Gier, Jr., and entitled Apparatus and Method for Forming Sinuous WireStructural and Heat Exchange Elements.

The heat exchanger described in the above identified patent is veryefiicient in relation to its size and weight. However, the ratio betweenthe amount of heat to be transferred by it and the minimum space andweight of the exchanger can be increased by the present improvements.

In the range of the mass velocities encountered in such installations asgas turbines, pin fin extended surfaces formed of pins of circularcross-section have a lower weight and volume for a given performancethan any extended surface heretofore known. Due to the high frictionfactors characteristic of a surface of this type, a large frontal areaexposed to the oncoming gases is combined with a short flow length. Dueto these characteristics, a design involving the principles of the abovepatent, while very effective for the heating or cooling of fluids ofhigh density, is not as effective for fluids of relatively low density.For example, in gas turbine applications, the hot gases of low densityflow through the heat exchanger and give up a great part of their heatto cooler high pressure air to be heated and expanded through theturbine. The present invention is particu- 2,892,618 Patented June 30,1959 larly applicable to the core passages through which the hot gasespass at low pressure.

A streamlined pin surface, such, for example, as disclosed in the aboveidentified patent, improves to some extent the results obtained underthe latter conditions, but it falls considerably short of optimumefficiency. It is true that the friction factors of the pins ofstreamlined cross-section are considerably reduced relative to that ofcircular pins--in fact, they may be as little as one-half that of pinsof circular cross-section. Also, the frontal area can be reduced in thecase of the pins of streamlined cross-section so that they can betterlend themselves to the counterflow design. However, these advantages ofstreamlined pins are offset to some extent by their lower heat transfercoefficient which is due partly to laminar or boundary layer effectscaused by the stream lined shape and the resultant carry over of thisefiect from one pin to the next succeeding pin in the direction of flow.This carry over of the laminar layer can be reduced by spacing thestreamlined pins a greater di-stance apart in the direction of flow.But, then, an increase in spacing defeats the desired reduction in thesize of the exchanger.

In accordance with the present invention, a pin arrangement is providedby virtue of which advantages due to a streamlined design of the pinsare obtained, and the disadvantages thereof are eliminated by providingstreamlined pins of different cross-sectional shape and aspect ratio,arranged in a row in the direction of fiow, so as to destroy or reducethe laminar layer and carry over thereof from pin to pin whileeliminating the need for increased spacing of adjacent pins in thedirection of How as a result of their having been streamlined. Furtheradvantages are obtained by an arrangement of the pins of one rowrelative to those of the next adjacent rows, laterally of the path offlow, so as to take better advantage of the surfaces of all of the pins.

Various objects and advantages of the present invention will becomeapparent from the following description wherein reference is made to thedrawings, in which,

Fig. 1 is a perspective View of a heat exchanger embodying theprinciples of the present invention;

Fig. 2 is a fragmentary top plan view of the heat exchanger illustratedin Fig. 1, parts thereof being shown in section, as indicated in Fig. 2by the line 2-2;

Figs. 3 and 4 are enlarged side elevation and top plan views,respectively, of a portion of the heat exchanger illustrated in Fig. 1,showing three adjacent rows of pin fins, and embodying the principles ofthe present invention;

Fig. 5 is a horizontal cross-sectional view taken on a line 5-5, in Fig.3;

Figs. 6, 7 and 8 are diagrammatic illustrations showing the manner offlow of gases past a series of pin fins, and illustrating some floweffects on which the present invention is based;

Fig. 9 is a top plan view illustrating a modified form of pins;

Figs. 10 and 11 are vertical cross-sectional views, taken on lines 10-40and 11-11, respectively, in Fig. 9;

Fig. 12 is a horizontal, cross-sectional view taken on a line 12-12, inFig. 10; and

Fig. 13 is a horizontal, cross-sectional view taken on a line 13-13 ofFig. 11.

Referring to the drawings, there is illustrated in Fig. 1 a heatexchanger or core of the general character described in the aboveidentified patent, and comprising a plurality of metal sheets 1 Whichare spaced apart flatwise from each other vertically. Arranged betweenthe sheets are a plurality of rows '2 of heat exchange elements in theform of upright streamlined pins 3 and 4, respectively, the pins 3 and 4of each row 2 being arranged alternatel-y relative to each other. Thepins 3 have a lower aspect ratio than the pins 4, aspect ratio being theratio of the major axis of the cross section of a pin to the minor axisof the cross section of the same pin. Also, in the present structure,the cross section of each pinhaving a higher aspect ratio than the crosssection of another pin of lesser aspect ratio has both a longer majoraxis and a shorter minor axis than the major axis and minor axis,respectively, of the cross section of the pin of lesser aspect ratio.

In referring to the pins as streamlined, it is not meant that they belimited to a true streamlined crosssection but that the cross-section isrounded at the leading end and the cross-section is longer than it iswide so as to reduce the resistance to flow as compared to pins ofcircular cross-section. Thus the cross-section may vary from a true andaccurate streamline to oval or elliptical. other so that each pin 3 ofone row lies between two pins 4 of the next adjacent rows between whichit is disposed, as more fully set forth hereinafter.

In the heat exchanger illustrated, alternate spaces between the sheets 1form passages for gases of different characteristics. For example,alternate passages may conduct the heated product from the turbine, andthe passages therebetween may conduct the air to be heated to theturbine. In the form illustrated, a group of alternate passages 5 areopen at the ends and closed at the sides by walls 6. Passages 7, betweenthe passages 5, are closed at the ends by walls 8 and open at the sides.Thus, in using the present exchanger with gas turbines, the hot lowpressure exhaust gases may be discharged through the passages 5 in thedirection indicated by the arrows A from end to end of the exchanger,and the high pressure air to be heated may pass through the passages 7transversely of the exchanger, as indicated by the arrows dB? Referringto Figs. 3 through 5, there are shown three rows of pins embodying theprinciples of the present intion. It is to be noted that thecross-sections of the pins 3 are wide relative to their length and thecross-sections of the pins 4 are narrow relative to their length. Asmentioned, the pins 3 and 4 are arranged in a row in alternate relationwith respect to each other in the direction of flow and are connected byligament portions 9 which are relatively thin and readily bendable attheir The rows 2 are offset endwise relative to each junctures with thepins 3 and 4, as described in the above identified patent.

The rows of pins in a given passage 5 or 7 are preferably arranged sothat each pin 3 lies between two pins 4 of the next adjacent rows,respectively, transversely of the direction of flow, and each pin 4 liesbetween two pins 3 of the next adjacent rows, respectively. The offsetpreferably is such that the axes of the pins which are in alignmenttransversely of the direction of flow lie in a common plane and areparallel to each other.

It is to be noted that the pins 3 and 4 all preferably comprise part ofa single length of wire of original constant cross-section, and thus thedifferent shapes of crosssections of the pins 3 as compared with thepins 4 is due to lateral redistribution of the metal of the wire. Ifdifferent ratios are desired, the wire may be upset at spaced portionsalong its length prior to forming the pins, so as to provide bothlongitudinal and lateral redistribution of the metal and a greater totalamount of metal in one shape of pin than in another.

Referring next to Figs. 6, 7 and 8, the reason for the shaping of thepins in this manner will be readily understood.

Referring first to Fig. 6, there is illustrated a plurality of pins 15arranged in rows in a manner hereinbefore described. All of these pins15 are of the same aspect ratio and all are streamlined or lenticular incross-section. The direction of flow is indicated by the arrow 16 andthe general pattern of the flowing gases is indicated by the lines 17and 18. As indicated by the dotted lines 18, there is a laminar layer ateach side of each pin which tends to carry over from one pin to the nextsucceeding pin in the row. These laminary layers cause a somewhatturbulent but low velocity area 19 to be formed between the trailingedge of one pin and the leading edge of the next adjacent pin in thedirection of flow. This low velocity area 19 lies between and is boundedby the dotted lines 18 and is indicated by the short crisscrossingcurved dash lines. This low velocity of flow causes a very lowcoefficient in the areas 19 and greatly reduces the etfectiveness of thefrontal areas of succeeding pins at the trailing ends of the areas 19.

Next, referring to Fig. 7, it is to be noted that if the pins 15 aremoved farther apart from each other in the row in the direction of flow,then the laminar layers from opposite sides of each pin draw moreclosely together, thus reducing the size and width of the low velocityareas 19. This allows the continuance of a higher velocity flow of thegases between the trailing edge of one pin to the leading edge of thenext adjacent pin in the direction of flow, but due to the greaterspacing of the pins from each other in the direction of flow, there is asmaller amount of heat exchange surface for a given volume of heatexchanger or core. The streamlining of the pin fins reduces the frictionfactor of the flow, the friction factor of the pins 15, for example,being only about half that for corresponding pins of circularcross-section. This permits the reduction of the frontal area of thepins 15 and lends itself to a counter flow design. However, the heattransfer coefficient of the streamlined pin fins is lower than that ofthe pin fins of circular cross-section, due partly to the laminarboundary layers, indicated at 18, at each side of streamlined pin finsbeing carried over from the leading pin to the next adjacent trailingpin. As shown, this condition can be improved, in turn, by increasingthe spacing of the pins in the flow direction, as shown in Fig. 7. Butincreasing the spacing in this manner reduces the amount of heatingsurface too greatly in relation to the volume of the heat exchanger.

Thus, Fig. 6 shows the desired spacing from the point of view ofcompactness, but in the structure there illustrated, the area at 19 isquite large.

In Fig. 7, the longitudinal spacing has been increased with reduction ofthe area 19 and some improvement in the conditions and results, but withtoo great a size and lack of compactness of the resultant core.

Referring next to Fig. 8, it is to be noted that, by alternating, in thedirection of flow in a given row, pins 15 of low aspect ratio with pins16 of high aspect ratio, and spacing the former sufiiciently far apartto cause the laminar layers at their opposite sides to approach moreclosely and thus reduce the areas 19 and maintain a higher velocity ofhow between the trailing edge of one pin and the leading edge ofanother, and introducing in the increased space between the low aspectratio pins 15, pins 16 of higher aspect ratio, the advantage of greaterspacing of the pins 15 is retained Without the corresponding increase insize of the core. Also, there is obtained a larger ratio of free airspace as compared to a structure in which the aspect ratio of all pinsis low, particularly since pins of high aspect ratio are staggeredtransversely of the rows with pins of smaller aspect ratio.

In Fig. 8, the spacing of pins 15 of lower aspect ratio has been doubledand pins 16, of higher aspect ratio have been introduced midway betweenthe two adjacent lower aspect ratio pins of the same row.

Thus, in Figs. 1 through 5, introduction of pins 4 of high aspect ratiobetween the adjacent pins 3 of small aspect ratio, in the direction offlow, does not change appreciably the advantageous flow pattern in Fig.7, but retains it, as illustrated in Fig. 8.

An additional advantage is obtained by offsetting adjacent rows in thedirection of flow so that each pin 3 is centered between two pins 4 ofthe adjacent rows and vice versa. The advantage is that the flow streamis given a sine motion over the heating surface. This assists inreducing or breaking up the boundary layers, and in reducing oreliminating their carry over from one pin to the following pin in thedirection of flow. It accomplishes this result without creating any highdegree of turbulence. The flow resistance, therefore, is not increasedappreciably over the rows of pins spaced widely apart, as in Fig. 7.

The ratio of free area to frontal area is also increased and thevelocity of flow is substantially constant. Dead pockets are eliminatedor reduced so that the heat transfer coefiicient is improved without areduction in the amount of heating surface in a given volume of core.

Referring next to Figs. 9 through 12, there are shown pins 20 ofpredetermined aspect ratio alternating with pins 21 of larger aspectratio. Each of the pins is further modified by shaping it so that itscross-section is reduced in size at its axial midportion, that is,midway between its ends. The cross-sectional areas of each pin graduallyincrease from the midportion outwardly toward each end. This is donepreferably by decreasing the width of the pins at the variouscross-sections and not the length, thus providing a larger aspect ratioat the axial midportion of each pin than at its ends. This increases thefree space area and also the effective surface for heat exchange withina given volume of heat exchanger core.

In summary, therefore, streamlining the pins to improve the elfect to beobtained and render the structure eflicient for counterflow, introducesthe objectionable carry over of laminar layers from one pin to the nextpin in the direction of flow and thus greatly enlarges the low velocityspace between adjacent pins and decreases the efiiciency of heatexchange. This carry over phenomenon can be eliminated by spacing thepins farther apart in the row without changing their cross-sections, butthe latter solution too greatly increases the size of the heatexchanger. By alternating pins 4 of high aspect ratio between adjacentpins 3 of lower aspect ratio, the advantages of streamlining areobtained and its disadvantages eliminated, thus decreasing the size ofcore needed for a given amount of heat transfer while obtaining a moreadvantageous flow and resultant higher efficiency.

Finally, by alternating the pins transversely of the row so that eachpin of higher aspect ratio is between two of lower aspect ratio, andvice versa, transversely of the direction of flow, so as to produce awave or sine motion to the gases flowing through the exchanger, betterflow and heat exchange characteristics are obtained.

Having thus described my invention, I claim:

1. In a heat exchanger, a plurality of rows of pin fins, the rows beingspaced apart transversely of the direction of flow through the exchangerand the pin fins in each row being spaced apart from each other in thedirection of flow, each pin fin having a different aspect ratio thanthose pin fins of the same row between which it is disposed, and thecross section of each pin fin which has a higher aspect ratio than otherpin fins of lesser aspect ratio has both a longer major axis and ashorter minor axis, respectively, than the major axis and minor axis ofthe cross section of each pin fin of lesser aspect ratio.

2. A heat exchange structure according to claim 1, wherein alternate pinfins in the same row have substantially the same aspect ratio as eachother and the pin fins therebetween have substantially the same aspectratio as each other and different from the aspect ratio of saidalternate pin fins.

3. A heat exchange structure according to claim 1, wherein the rows arearranged so that each one of the pin fins of one row is aligned,transversely of the rows, with a pin fin of each of the adjacent rowsbetween which said one row is disposed, and the said one pin fin has anaspect ratio different from those between which it is disposedtransversely of the rows.

4. A heat exchange structure according to claim 1, wherein at least someof said pin fins are streamlined.

5. In a heat exchanger, a plurality of rows of pin fins, the rows beingspaced apart transversely of the direction of flow through the exchangerand the pin fins in each row being spaced apart from each other in thedirection of fiow, some of said pin fins of the same row having aspectratios difierent from each other, and the cross section of each pin finwhich has a higher aspect ratio than other pin fins of lesser aspectratio has both a longer major axis and a shorter minor axis,respectively, than the major axis and minor axis of the cross section ofeach pin fin of lesser aspect ratio.

6. A sinuous wire element for the purposes described and comprising alength of wire having spaced ligament portions which are wide and thinrelative to leg portions therebetween and readily bendable at theirjunctures with the leg portions, said wire being in sinusoidal form withthe ligament portions providing the crests of the waves and the legportions forming the sides of the waves, each leg portion beingstreamlined in the same direction, and the aspect ratio of each legportion being different from the aspect ratios of those leg portionsbetween which it is located, respectively, and the cross section of eachleg portion which has a higher aspect ratio than other leg portions oflesser aspect ratio has both a longer major axis and a shorter minoraxis, respectively, than the major axis and minor axis of the crosssection of each leg portion of lesser aspect ratio.

References Cited in the file of this patent UNITED STATES PATENTS2,678,808 Gier May 18, 1954

